Date:   Wed, 17 Jun 2020 17:28:57 +0200
From:   Niklas Söderlund 
        <niklas.soderlund@...natech.se>
To:     Michael Rodin <mrodin@...adit-jv.com>
Cc:     Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-renesas-soc@...r.kernel.org,
        linux-kernel@...r.kernel.org, michael@...in.online,
        efriedrich@...adit-jv.com, erosca@...adit-jv.com,
        Steve Longerbeam <steve_longerbeam@...tor.com>
Subject: Re: [PATCH] media: rcar-vin: Move media_device_register to async
 completion

Hi Michael,

On 2020-06-17 17:15:37 +0200, Michael Rodin wrote:
> Hi Niklas and Steve,
> 
> On Wed, Jun 17, 2020 at 12:56:46PM +0200, Niklas Söderlund wrote:
> > Hi Michael and Steve,
> > 
> > On 2020-06-16 19:31:36 +0200, Michael Rodin wrote:
> > > From: Steve Longerbeam <steve_longerbeam@...tor.com>
> > > 
> > > The media_device is registered during driver probe, before async
> > > completion, so it is possible for .link_notify to be called before
> > > all devices are bound.
> > > 
> > > Fix this by moving media_device_register() to rvin_group_notify_complete().
> > > This ensures that all devices are now bound (the rcar-csi2 subdevices and
> > > and video capture devices) before .link_notify can be called.
> > 
> > I'm curious to what situation created the need for this change. I'm 
> > currently trying to take the VIN driver in the opposite direction [1] 
> > with the end goal of registering video devices at probe time and then 
> > allow the media graph to populate as devices becomes available.
> 
> It looks like almost all platform drivers call media_device_register() in
> the completion callback. From my understaning it is necessary to ensure
> that all subdevices are bound and all links are created before the user
> can enable any link (which would trigger link_notify callback execution)
> and set formats. If I am not mistaken, Steve could observe an "OOPS" or
> at least it is theoretically possible.

If an OOPS have been observed I would be interested to see it. That way 
we can fix the OOPS and keep the media graph registration where it is 
today.

> 
> Actually I found that this patch alone is not enough even if it is correct,
> because we also have to register the media device in rvin_parallel_notify_complete()
> in case if there is only a parallel video input device attached.
> 
> > My reason for this is that we could have a functional pipeline inside 
> > the graph even if it's not complete. This came out of the GMSL work done
> > a while pack where I had a faulty camera that would prevent the other 7 
> > in the system to function.
> 
> I agree that if a probe of a faulty subdevice fails, this should not affect
> functionality of the other attached subdevices. The "complete" callback of
> the async notifier is probably not executed in this case, so I guess, we
> would have to register the media device in the "bound" callback after the first
> subdevice has been probed? Otherwise there is not much sense to have video
> capture devices, which are not connected to any source.

Calling it in the bound callback is mostly the same as it is today, as 
link_notify could then be called when not all entities are in the graph.  
In fact even if we where tp move the media device registration to the t
complete callback we have this problem if any of the subdevices are 
unbound. Then we are back to the state with a registerd media device 
where not all entities are present.

I think the solution here is to address the issue (if any) in the 
link_notify callback when the graph is not fully populated.

> 
> (Delayed) population of the media graph after media device registration
> sounds also like a requirement for device tree overlay support, which would
> also be a nice feature.
> 
> > 1. [PATCH 0/5] media-device: Report if graph is complete
> > 
> > > 
> > > Signed-off-by: Steve Longerbeam <steve_longerbeam@...tor.com>
> > > Signed-off-by: Michael Rodin <mrodin@...adit-jv.com>
> > > ---
> > >  drivers/media/platform/rcar-vin/rcar-core.c | 14 ++++++--------
> > >  1 file changed, 6 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/drivers/media/platform/rcar-vin/rcar-core.c b/drivers/media/platform/rcar-vin/rcar-core.c
> > > index 7440c89..e70f83b 100644
> > > --- a/drivers/media/platform/rcar-vin/rcar-core.c
> > > +++ b/drivers/media/platform/rcar-vin/rcar-core.c
> > > @@ -253,7 +253,6 @@ static int rvin_group_init(struct rvin_group *group, struct rvin_dev *vin)
> > >  	struct media_device *mdev = &group->mdev;
> > >  	const struct of_device_id *match;
> > >  	struct device_node *np;
> > > -	int ret;
> > >  
> > >  	mutex_init(&group->lock);
> > >  
> > > @@ -266,7 +265,6 @@ static int rvin_group_init(struct rvin_group *group, struct rvin_dev *vin)
> > >  	vin_dbg(vin, "found %u enabled VIN's in DT", group->count);
> > >  
> > >  	mdev->dev = vin->dev;
> > > -	mdev->ops = &rvin_media_ops;
> > >  
> > >  	match = of_match_node(vin->dev->driver->of_match_table,
> > >  			      vin->dev->of_node);
> > > @@ -278,11 +276,7 @@ static int rvin_group_init(struct rvin_group *group, struct rvin_dev *vin)
> > >  
> > >  	media_device_init(mdev);
> > >  
> > > -	ret = media_device_register(&group->mdev);
> > > -	if (ret)
> > > -		rvin_group_cleanup(group);
> > > -
> > > -	return ret;
> > > +	return 0;
> > >  }
> > >  
> > >  static void rvin_group_release(struct kref *kref)
> > > @@ -688,6 +682,8 @@ static int rvin_group_notify_complete(struct v4l2_async_notifier *notifier)
> > >  		return ret;
> > >  	}
> > >  
> > > +	vin->group->mdev.ops = &rvin_media_ops;
> > > +
> > >  	/* Register all video nodes for the group. */
> > >  	for (i = 0; i < RCAR_VIN_NUM; i++) {
> > >  		if (vin->group->vin[i] &&
> > > @@ -736,8 +732,10 @@ static int rvin_group_notify_complete(struct v4l2_async_notifier *notifier)
> > >  		}
> > >  	}
> > >  	mutex_unlock(&vin->group->lock);
> > > +	if (ret)
> > > +		return ret;
> > >  
> > > -	return ret;
> > > +	return media_device_register(&vin->group->mdev);
> > >  }
> > >  
> > >  static void rvin_group_notify_unbind(struct v4l2_async_notifier *notifier,
> > > -- 
> > > 2.7.4
> > > 
> > 
> > -- 
> > Regards,
> > Niklas Söderlund
> 
> -- 
> Best Regards,
> Michael

-- 
Regards,
Niklas Söderlund

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